The present invention relates to a solid state relay, and more particularly to a novel solid state relay in which less EMI noise leaks in to a power source, and which can be produced without especially increasing the cost and the size as compared with a relay of the conventional art.
Usually, a solid state relay is configured so that an input circuit and an output circuit are electrically insulated from each other by a photocoupler, and a main switching element (thyristor, triac (triode bilateral thyristor), or the like) interposed in the output circuit is operated in accordance with an electric signal applied to the input circuit, thereby closing or opening a load connected to the output circuit.
As a solid state relay of the conventional art, for example, known are a first conventional art example shown in FIG. 8, a second conventional art example shown in FIG. 9, and a third conventional art example shown in FIG. 10. In the solid state relay of the first conventional art example, a trigger system is employed in which the main switching element is triggered by a triac coupler. In the solid state relay of the second conventional art example, a trigger system is employed in which the main switching element is triggered by a diode bridge and a thyristor. In the solid state relay of the third conventional art example, a trigger system is employed in which the main switching element is triggered by a diode bridge and a thyristor coupler. These three trigger systems can be realized at a low cost, and hence are used in many solid state relays.
FIG. 11A shows a connection example in the case where the solid state relays shown in FIGS. 8, 9, and 10 are used. In the figure, 1109 denotes a solid state relay (SSR), 1107 and 1108 denote input terminals of the solid state relay, 1104 and 1103 denote load terminals of the solid state relay (and serving also as a pair of external connection terminals which are conductive with the ends of the incorporated main switching element), 1111 denotes an input signal voltage in this case it is a symbol for a DC power source, 1122 denotes a load which is to be driven, and 1123 denotes an AC power source for driving an external load.
The details of the solid state relays shown in FIGS. 8, 9, and 10 will be sequentially described.
In the first conventional art example shown in FIG. 8, a triac (triode bilateral thyristor) 816 serving as a main switching element is triggered by a photo-triac(triode bilateral thyristor) coupler 813. In the figure, 807 and 808 denote a pair of input terminals to which the input signal voltage 1111 (see FIG. 11A) is supplied, 812 denotes an input circuit which functions as a buffer or the like for an input signal, 813 denotes the photo-triac coupler (configured by optically coupling a light emitting diode 814 with a photo-triac 815) which electrically insulates the input circuit 812 and an output circuit from each other, 816 denotes a power triac incorporated into the output circuit and serving as a main switching element, 817 denotes a current limiting resistor for the photo-triac 815, 818 denotes a gate bias resistor for the power triac 816, a resistor element 819 and a capacitor element 820 are connected in series to constitute a surge absorbing circuit, and 804 and 803 denote a pair of external connection terminals which are conductive with the ends of the triac serving as the main switching element.
The operation in the case where the solid state relay of the first conventional art example is employed as the solid state relay (SSR) 1109 shown in the connection diagram of FIG. 11A will be described with reference to FIGS. 11B and 11C.
The relationship between the load current I and the voltage VT between the terminals 803 and 804 during the operation of the solid state relay are indicated by the broken and solid lines in the waveform chart of FIG. 11B. In this case, a resistance load is as the load 1122. In the figure, when the terminal-to-terminal voltage VT reaches a specified on-start voltage VON in accordance with the voltage of the power source 1123 as indicated by the sold line in the figure the power triac 816 serving as the main switching element is turned on, thereby causing the load current I to start flowing through the load 1122. At the same time, the terminal-to-terminal voltage VT instantly drops to a specified power-chip ON voltage (ON-state voltage) VTMP.
The change of the terminal-to-terminal voltage (voltage between the terminals 803 and 804) VT before and after turning on the power triac 816 is enlarged and shown in FIG. 11C. As shown in the figure, the on-start voltage VON that is equal to the terminal-to-terminal voltage VT at which the triac 816 is triggered (hereinafter, indicated as VON1 in the first conventional art example) is determined by the sum of the voltage drop (IGxc3x97RGS) of the current limiting resistor 817 due to the operation current IG of the photo-triac 815, the on-state voltage VTM1 of the photo-triac 815, and the voltage drop VGT of the resistor 818 as indicated by following expression (Eq. 1):
VON1=IGxc3x97RGS+VTM1+VGT.xe2x80x83xe2x80x83(Eq. 1)
As apparent from this, in the trigger circuit of the first conventional art example, the on-state voltage VTM1 of the photo-triac 815 which serves as a triggering element is high, and hence the on-start voltage VON1 at which the triac 816 which serves as the main switching element is triggered is inevitably made higher. This causes a problem in that the noise terminal voltage is high.
Next, in the second conventional art example shown in FIG. 9, a triac(triode bilateral thyristor) 916 serving as a main switching element is triggered by using a diode bridge 917 and a thyristor 922. In the figure, 907 and 908 denote a pair of input terminals to which the input signal voltage 1111 (see FIG. 11A) is supplied, 912 denotes an input circuit which functions as a buffer or the like for an input signal, 913 denotes a phototransistor coupler (configured by optically coupling a light emitting diode 914 with a phototransistor 915) which electrically insulates the input circuit 912 and a trigger circuit 921 in an output circuit from each other, 916 denotes a power triac incorporated into the output circuit and serving as a main switching element, 917 denotes the diode bridge which rectifies the power source voltage and then applies the rectified voltage to a thyristor 922, 922 denotes the thyristor which is triggered by the trigger circuit 921 to trigger the main switching element, 918 denotes a gate bias resistor for the power triac 916, a resistor element 919 and a capacitor element 920 are connected in series to constitute a surge absorbing circuit, and 904 and 903 denote a pair of external connection terminals which are used to lead out the ends of the triac which serve as the main switching element to the outside.
Also the operation in the case where the solid state relay of the second conventional art example is employed as the solid state relay (SSR) 1109 shown in the circuit diagram of FIG. 11A which will be described with reference to FIGS. 11B and 11C in the same manner as the first embodiment. Also in the second embodiment, the on-start voltage VON that is equal to the terminal-to-terminal voltage VT at which the triac 916 is triggered in the same manner as the first conventional art example (hereinafter, indicated as VON2 in the second conventional art example) is determined by the sum of the on-state voltage VTM2 of the thyristor 922, the on-state voltage (2xc3x97VF) of diodes in the diode bridge 917, and the voltage drop VGT of the resistor 918 as indicated by following expression (Eq. 2):
VON2=VTM2+2xc3x97VF+VGT.xe2x80x83xe2x80x83(Eq. 2)
As apparent from this, in the trigger circuit of the second conventional art example, the on-state voltages VTM2 and VF of the elements (the thyristor 922 and the diode bridge 917) are high, and hence also the on-start voltage VON2 to trigger the triac 916 which serves as the main switching element is inevitably made higher. This causes a problem in that the noise terminal voltage is high.
Next, in the third conventional art example shown in FIG. 10, a triac(triode bilateral thyristor) 1016 serving as a main switching element is triggered by using a diode bridge 1017 and a photo-thyristor coupler 1013. In the figure, 1007 and 1008 denote a pair of input terminals to which the input signal voltage 1111 (see FIG. 11A) is supplied, 1012 denotes an input circuit which functions as a buffer or the like for an input signal, 1013 denotes the photo-thyristor coupler (configured by optically coupling a light emitting diode 1014 with a photo-thyristor 1015) which electrically insulates the input circuit 1012 and an output circuit from each other, 1016 denotes a power triac that is incorporated into the output circuit and serves as a main switching element, 1017 denotes the diode bridge which rectifies the power source voltage and then applies the rectified voltage to the photo-thyristor 1015, a circuit configured by a resistor 1021 and a capacitor 1022 is a firing angle control circuit for the photo-thyristor, 1018 denotes a gate bias resistor for the power triac 1016, a resistor element 1019 and a capacitor element 1020 are connected in series to constitute a surge absorbing circuit, and 1004 and 1003 denote a pair of external connection terminals which are used to lead out the ends of the triac which serves as the main switching element to the outside.
Also, the operation in the case where the solid state relay of the third conventional art example is employed as the solid state relay (SSR) 1109 as shown in the circuit diagram of FIG. 11A will be described with reference to FIGS. 11B and 11C in the same manner as the first embodiment. Also in the third embodiment, the on-start voltage VON that is equal to the terminal-to-terminal voltage VT at which the triac 1016 is triggered in the same manner as the first conventional art example (hereinafter, indicated as VON3 in the third conventional art example) is determined by the sum of the on-state voltage VTM3 of the photo-thyristor 1015, the on-state voltage (2xc3x97VF) of diodes in the diode bridge 1017, and the voltage drop VGT of the resistor 1018 as indicated by following expression (Eq. 3):
VON3=VTM3+2xc3x97VF+VGT.xe2x80x83xe2x80x83(Eq. 3)
As apparent from this, in the trigger circuit of the third conventional art example, the on-state voltages VTM3 and VF of the elements (the photo-thyristor 1015 and the diode 1017) are high, and hence the on-start voltage VON3 to trigger the triac 1016 serving as the main switching element is triggered is inevitably made higher. This causes a problem in that the noise terminal voltage is high.
Although the above three examples in which a main switching element is a triac(triode bilateral thyristor) have been described, the problem of a high noise terminal voltage (EMI noise) similarly occurs also in the case where a main switching element is a thyristor.
As apparent from the waveform chart shown in FIG. 11C, it will be seen that the noise terminal voltage can be reduced by making the on-start voltage VON to trigger the triac(triode bilateral thyristor) serving a main switching element and the power chip-on voltage (on-state voltage) VTMP closer to each other. One technique to accomplish the above, it may be contemplated to raise the power chip-on voltage VTMP. However, the rise of the power chip-on voltage VTMP corresponds to increase of heat generation (loss) of a power element, and hence cannot be employed when a large current flows is used. As another technique, it may be contemplated to lower the power chip-on voltage VTMP. In order to realize this, for example in the case of the triac(triode bilateral thyristor) coupler system (the first conventional art example; see FIG. 8), it seems that from the viewpoint of a main switching element, a triac (thyristor) with a low operation current IG and a low voltage drop VGT of a resistor is to be selected. However, such a circuit configuration is always expensive. Moreover, since the current limiting resistor 817 is a protective resistor, the reduction of the resistance RGS is limited. The remaining way is to lower the on-state voltage VTM1 of the photo-triac 815 in the triac (triode bilateral thyristor) coupler 813. However, the lowering of the on-state voltage VTM1 is also limited because a photo-triac is inherently a semiconductor element.
On the other hand, Japanese Patent Publication (Kokai) No. HEI7-226130 xe2x80x9cHigh current solid state AC relay with low EMI emissionxe2x80x9d proposes a solid state relay that comprises a pre-driver using a pair of MOSFETs in which the source terminals are connected to each other, whereby EMI noises can be reduced to a negligible level. However, also such a solid state relay cannot attain a small effect without a large increased device cost due to replacement and addition of circuit components.
Under such circumstances, a technique is proposed in which no countermeasure against noise is taken in a solid state relay, and a filter is added to the outside of the solid state relay, whereby the noise terminal voltage is lowered (EMI noises are solved). FIG. 12 shows a noise reduction circuit for a solid state relay having a typical EMI filter. As shown in that FIG. 12, the noise reduction circuit has a configuration in which is based on the circuit configuration shown in FIG. 11A, an EMI filter 1201 wherein capacitors 1202 and 1203 are respectively connected across the ends of a coil 1204 is connected between a load 1122 and a power source 1123. In FIG. 12, the same components as those of FIG. 10 are denoted by the same reference numerals, and their description is omitted.
When the EMI filter 1201 is disposed outside the solid state relay as above, a steep change of the power which causes noises can be filtered by the EMI filter 1201, and hence a improved noise reducing effect can be attained. However, in the case where the EMI filter is an LC filter as shown in FIG. 12, the resistance component of the coil 1204 generates a large amount of heat, and in the case where the filter is an RC filter, the incorporated resistor also generates a large amount of heat, with the result that the energy loss in the EMI filter is large. When using a solid state relay with a large current flow, a measure for increasing the wire diameter of the coil 1204 must be taken in order to lower the resistance. As a result, there arises a disadvantage in that the size of the EMI filter is extremely increased.
In a conventional solid state relay in which a thyristor or a triac (triode bilateral thyristor) is used as a main switching element, factors which determine the on-start voltage (VON1, VON2, VON3) at which the main switching element is turned on include elements such as a triac, a thyristor, and a photo-thyristor having anon-start voltage (VTM1, VTM2, VTM3) that is relatively high. Therefore, the on-start voltage (VON1, VON2, VON3) of the main switching element is increased. This causes a problem in that the EMI noise level (noise terminal voltage) is inevitably made higher.
In a solid state relay using such a trigger system based on a triac, a thyristor, a photo-thyristor, or the like, it may be contemplated to take a countermeasure in order to lower the on-start voltage of a main switching element causing EMI noises in which a coupler of a low on-start voltage (VTM), or an element of a low voltage drop (VGT) is used. However, an element of a low on-start voltage (VTM) or a low voltage drop (VGT) is expensive, and the lowering of the voltage is limited because of the structure of the element. Therefore, EMI noises cannot be sufficiently reduced by such an internal improvement.
On the other hand, in the technique in which the EMI noise level (noise terminal voltage) is lowered by externally adding a filter, a steep change of the power can be filtered, and a large noise reducing effect can be attained. However, the technique has problems in that the filter size is large, the cost is high, the connection is inconvenience, and the heat generation is large.
The invention has been conducted in view of the problems of the conventional art. It is an object of the invention to provide a solid state relay in which less EMI noise (noise terminal voltage) is generated, and which can be produced without especially increasing the cost and the size as compared with a relay of the conventional art.
A first feature of the invention is a solid state relay that has a first external connection terminal which is conductive with one end of a main switching element, and a second external connection terminal which is conductive with another end of the main switching element, and is used with a load and a power source connected in series between the external connection terminals, wherein a third external connection terminal is disposed, the third external connection terminal is connected to the second external connection terminal via a capacitor, the load and the power source are connected in series between the first external connection terminal and the second external connection terminal, and a node of the load and the power source is connected to the third external connection terminal, thereby allowing a resistance of the load and an electrostatic capacitance of the capacitor to constitute an RC filter circuit for preventing noise leakage.
According to the first feature, the load and the power source are sequentially connected in series between the first external connection terminal and the second external connection terminal, and the node of the load and the power source is connected to the third external connection terminal. When the load is seen from the second external connection terminal and the third external connection terminal, therefore, an RC low-pass filter for preventing noise leaking is formed by the resistance component (R) of the load and the electrostatic capacitance (C) of the incorporated capacitor. As a result, even when a solid state relay of any type is used, the EMI noise level (noise terminal voltage) can be lowered.
Since the resistance component of the load is used in the filter, a radiator for the filter is not required. When the load is a heater or the like, particularly, the heat generated in the filter can be used directly as heating energy, and hence the relay is economical.
It is required only to dispose the new third external connection terminal, and internally connect the capacitor between the terminal and the second external connection terminal. Therefore, the cost and the size are not especially increased as compared with a relay of the conventional art.
A second feature is that the connection of the third external connection terminal and the second external connection terminal via the capacitor is made inside a body of the solid state relay.
According to the second feature, the connection of the third external connection terminal and the second external connection terminal via the capacitor is performed inside the body of the solid state relay. Unlike the case where the connection between the third external connection terminal and the second external connection terminal is performed outside of the body of the relay via a capacitor, therefore, an extra space for placing the capacitor is not necessary in the vicinity of a terminal block of the solid state relay, and the external appearance is not impaired by such a space.
A third feature is that the solid state relay has a capacitor mounting structure which allows the electrostatic capacitance of said capacitor to be variable.
According to the third feature, the solid state relay has a capacitor mounting structure which allows the electrostatic capacitance of the capacitor to be variable in accordance with the resistance of the external load. When the electrostatic capacitance of the capacitor is changed in accordance with the resistance of the external load so that the time constant RC is substantially constant, therefore, the frequency characteristic of the low-pass filter can always be set to an optimum value.
A fourth feature is that the capacitor mounting structure which allows the electrostatic capacitance of the capacitor to be variable is formed so that a whole or apart of the electrostatic capacitance of the internally connected capacitor is shared by one, two, or more capacitor units, and each of the capacitor units is detachable with respect to the body of the solid state relay via a connector.
According to the fourth feature, the capacitor mounting structure allows the electrostatic capacitance of the capacitor to be variable in accordance with the resistance of the external load. Furthermore, a configuration is employed in which the whole or apart of the electrostatic capacitance of the internally connected capacitor is shared by one, two, or more capacitor units, and each of the capacitor units is detachable with respect to the body of the solid state relay via a connector. Therefore, the electrostatic capacitance can be changed by performing only replacement or addition of the capacitor units, and without requiring wiring labor. Noise reduction suitable for the usage conditions can be easily realized.
A fifth feature is that the capacitor mounting structure which allows the electrostatic capacitance of the capacitor to be variable is formed so that a whole or apart of the electrostatic capacitance of the capacitor is made by a variable capacitance capacitor device.
According to the fifth feature, the capacitor mounting structure allows the electrostatic capacitance of the capacitor to be variable in accordance with the resistance of the external load. Furthermore, the whole or a part of the electrostatic capacitance of the internally connected capacitor is made by the variable capacitor device of a continuously variable type or a stepwise variable type. Therefore, the electrostatic capacitance can be changed by making only an adjustment or switching operation of the capacitor device, and without requiring wiring labor. Noise reduction suitable for the usage conditions can be more easily realized.